'There's a lot of looting going on in pharmacies, but to no avail.The drugs are being distributed in convoys, with military jeeps in front and behind. Masks costing a dollar are being sold on street corners for $20. E-mailed ads for counterfeit drugs are filling up my inbox.'
Nature magazine, May 2005, predicting what may happen during an outbreak of avian flu

It sounds the stuff of science fiction; to even describe such a breakdown of society is to invite accusations of alarmist hype and scaremongering. But this imaginary account of an American woman in the middle of a deadly flu pandemic was published last spring in the highly respected scientific journal Nature, as part of a special edition it produced on the threat. It wanted to look at how a nation would cope from the very first outbreak to the peak of the disease.

Last week the threat felt ever more real in Britain when it emerged that avian flu had moved out of South East Asia and infected 2,000 birds in Turkey. Yesterday it was announced by the Veterinary Laboratory Agency in Weybridge that the lethal strain - H5N1 - had marched across Europe's border, killing birds in Romania.

Is it time to panic? Is bird flu's transference to humans inevitable? Will hundreds of thousands die in Britain? Will millions die around the world? How is the government dealing with it? Is there a cure? What should we be doing now?

As the nation waits this weekend, with headlines stating that one of the most dangerous disease outbreaks of the last 100 years is imminent, one man holds some of the answers. Sir Liam Donaldson, Chief Medical Officer and the man charged with advising the government on health, says it is time to take notice.

At the moment the H5N1 virus remains a disease of birds that can jump to a person but not spread further, so limiting its impact. But experts are agreed that, with each occurrence of the deadly flu in the bird population, the chance that it will mutate into something that could jump from human to human increases. Carried at speed by cross-country travel, it would quickly spread through the world population.

Donaldson, sitting in his office at the Department of Health in Whitehall, explains the probable scenarios. A flu pandemic is a biological inevitability, he says.

'It is public health enemy number one,' Donaldson said. 'It is at the top of our priority list.'

Donaldson said a flu pandemic was expected every 10 to 30 years, and as the last one happened in 1968 we were long 'overdue'.

On Thursday, Donaldson will present an updated version of the British emergency contingency plan that estimates that a flu pandemic would infect one in four of the population, killing 53,000 people in Britain and millions worldwide. The plans have been made around the 'most likely scenario', according to Donaldson, but officials at the Civil Contingency Secretariat of the cabinet have warned that, at worst, deaths could rise much higher - up to 700,000.

The flu is expected to have an incubation period of one to three days and doctors will be aiming to get the antiviral drug, Tamiflu, to patients early to reduce hospitalisations.

The emergency plans include gathering a stockpile of the anti-flu drugs to treat 14.6 million people. The target will not be hit before next September.

'If we can buy time we will be in a stronger position,' he said. The widespread media coverage was not 'hysteria', he added, and was much needed to inform the country of the threat: 'We want to raise awareness because it is a natural biological phenomenon. You get pandemics because [flu viruses] are constantly mutating - usually to a small extent, but from time to time you get a very big shift.'

Donaldson pointed to the flu pandemic of 1918. Then a lethal flu virus flashed around the globe, infecting one in three people worldwide and killing one in 33 of those it hit. The global death toll has been estimated at 50 million. The virus struck down the young and healthy as well as the elderly.

'We are better equipped to deal with it now, because we didn't have antivirals [even in] 1968,' said Donaldson. 'We didn't have such good knowledge and vaccine production was less sophisticated.' Most people will endure a nasty illness and recover, he added, but said: 'What we can't do is make it go away completely.'

Donaldson said he doubted Britain would have to close airports or use quarantine. The speed of the spread would make them unworkable strategies, he said, but he would advise those suffering to stay at home and not travel.

Across Britain, doctors will soon be gearing up, setting in place their own emergency plans. At its peak GPs covering a population of 100,000 can expect to see 1,000 extra patients every week. The Department of Health is expected to announce that doctors should try to treat more people in the community to free hospital beds, and practice nurses will be brought in to help.

Doctors are now admitting they know too little about the disease and have not been informed about how to prepare for the increased workload. The majority feel ill-equipped to reassure patients.

Some doctors reported rushes of patients asking about the antiviral vaccine. There is also confusion about whether the conventional flu jab will protect against avian flu. There is a fair chance that it will not, but the NHS wants to limit 'normal flu' in the population because if the bird flu mixes with a common strain it could produce a hybrid that could spark a pandemic.

How did we get here? Bird flu is not a new threat. It was in 1997 that Hong Kong discovered that a bird flu virus had infected 18 citizens. They averted a pandemic by taking astonishingly swift action, culling more than 1.5m ducks, geese and hens, disinfecting poultry markets and banning imports.

Albert Osterhaus, professor of virology at the Erasmus Medical Centre in Rotterdam, who has been tracking the virus around the world said: 'What they showed us was what everyone until then had thought impossible - that the virus could leap directly from birds to humans.'

Next it was discovered that it was being harboured in ducks in southern China and since then the virus has spread to poultry in nine Asian countries. At the end of 2003 and the beginning of 2004, the farmers and families who live alongside the birds in Vietnam and Thailand started to pick up the infection.

Many of the victims did not receive proper antiviral medication until it was too late. By the time they were put on respirators, their lungs were already being broken down and were slowly filling with fluid.

The key point is that the virus cannot yet spread from human to human and so its human victims are limited. The great danger is that, as it mutates between bird species and also between different species, it will change in a way that will make it easy to transmit between people.

'The fact it has arrived in Romania and Turkey doesn't raise the risk any more than had it been two countries in South Asia,' said Dr Karl Nicholson, professor of infectious diseases at the University of Leicester. 'The spread to Europe is just a continuing part of the march of the virus and not unexpected. This looks like a particularly virulent type of virus and is likely to reoccur.'

With each reappearance the opportunity for a mutating are increased, added Nicholson. First there is the risk that the bird flu virus mixes with a normal human virus. Or, worse, the virus will gradually mutate into a newer strain that enables the dangerous proteins within the virus to stick better to human lung cells. The process, known as 'antigenic shift', was what happened in 1918.

'The first scenario could occur at any moment,' added Nicholson. H5 is already showing itself to be a ruthless killer. Of those infected in South East Asia, 50 per cent have been killed.

Normal strains of flu tend to take lives because the virus multiplies in the airways and can lead to complications in groups with weak hearts, such as the elderly. But H5 is acting differently. 'It is much more complex,' said Nicholson. 'The infection seems to damage organs, triggering an immune response and that response appears to be doing the damage. With H5 young children are dying, so something different must be occurring.'

'It could spread very quickly,' said John Oxford, professor of virology at the London School of Medicine. 'It will probably become less deadly, but much, much more infectious. What is happening in Europe is a sideshow issue, it is still much more likely to [mutate into a pandemic] in Asia, and that will be the epicentre.'

But whether the core is in Asia or Eastern Europe, it will make little difference, as wherever it starts it will be impossible for Britain to close its doors to the disease.

Already the Health Protection Agency (HPA) has carried out a number of dry runs about what they will do in the event of an outbreak - setting up simulation nerve centres in London, Leeds and the East Midlands. Meanwhile, doctors have been told to refer any patients in Britain with symptoms of a bird flu-like virus who have travelled to any of the infected areas to the HPA headquarters to be tested.

So far there has not been a rush on antivirals in Britain, although it is happening in Switzerland, France and Italy.

The French Prime Minister, Dominique de Villepin, asserted on Friday that people 'should not give in to panic'. But all his statement did was spark a rush for bird flu remedies. 'Tamiflu? There is none. I could sell 20 packets a day if I wanted to,' said Parisian pharmacist Helene Cohen.

Back at Richmond House, Donaldson was calm about the threat. Though it would sadly take millions of lives worldwide, it was part of a natural cycle and flu pandemics had existed throughout history and would continue into the future.

'This is not a Frankenstein disease,' he said. Unlike terror attacks, it would not come as a surprise, he added, and unlike earthquakes and hurricanes it would not destroy the country's infrastructure: 'Flu pandemics are a natural phenomenon, not a natural disaster.'